As a means for charging materials to be charged such as photosensitive materials in image forming apparatuses of the electrophotographic recording system, the static recording systems, i.e., the contact charging system wherein rolls for electrostatic charging are rotated in contact with the surfaces of the photosensitive material to be charged has recently attracted attention, because this system has the advantages of smaller generation of ozone and lower applied voltage than the corona discharging system.
Such a roll fundamentally comprises a metallic core member and a conductive elastomer layer formed thereon.
However, in the case of the rolls for electrostatically charging this single-layer structure, adding conductive materials lowers the elasticity of elastomers such as EPDM (ethylene-propylene-diene) rubber, urethane rubber and silicone rubber. It is therefore necessary to lower the hardness of these rubbers. For this reason, softening agents, such as paraffin oil or plasticizers or sulphur-based crosslinking agents are added to the elastomer members, causing them to bleed or bloom, respectively, thereby staining the surfaces of the photosensitive materials to be charged and hindering the formation of images.
For the purpose of preventing the above-mentioned bleeding and blooming, for example, a roll for electrostatic charging in which a conductive surface layer composed of a conductive resin composition mainly comprising a methoxy-modified polyamide, etc. is formed on a surface of a conductive elastomer layer. This concept has been proposed in JP-A-64-66674 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
However, even when such a surface layer is provided, the conductive elastomer layer formed of EPDM rubber fails to have a sufficient sealing effect against oil-based liquid matter to prevent the exuding phenomenon under high temperature and high humidity. Further, the methoxy-modified polyamide is easily affected by humidity, and has the disadvantages of generating leaks due to pinholes in the photosensitive material under the circumstances of high humidity and of generating poor electrostatic charge under the circumstances of low humidity.
Rolls for electrostatic charging provided with conductive surface layers (volume resistivity: 10.sup.4 to 10.sup.12 .OMEGA..cm) formed of methoxy modified nylon or urethane have also been proposed (JP-A-6A-20518, JP-A-3-233473, etc). However, their resistance is seriously affected, particularly by changes in atmospheric humidity, and they can increase three times or more in volume resistivity under low humidity, which makes providing a stable, uniform electrostatic charge impossible.
Furthermore, a roll for electrostatic charging having the multi-layer structure in which a conductive elastomer layer, is divided into an elastic layer and a conductive layer and a resistive layer (volume resistivity: 10.sup.6 to 10.sup.12 .OMEGA..cm) is provided on the conductive layer has also been proposed (JP-A-1-211779). However, similar to the above-mentioned previous proposals, the presence of the resistive layer such as nylon results in preventing stable, uniform electrostatic charge because of a wide fluctuation in resistance due to humidity and fails to sufficiently prevent leaking through pinholes in photosensitive material under high applied voltage.
On the other hand, in manufacturing rolls for electrostatic charging having the multi-layer structure consisting of 3 or more layers, as well as the above-mentioned rolls of the two-layer structure, the respective layers on the elastic material are usually formed by spray coating or dip coating. Using these coating processes, however, makes it difficult to ensure that the coated films are produced with a uniform thickness. As a result, it is difficult to ensure uniform electrostatic charge and stable resistance, and the many coating steps complicate the manufacturing procedures to make it difficult to reduce costs. In the case of rolls for electrostatic charging of the multi-layer structure in which the respective layers have different functions, coating may be repeated several times in each coating step in order to secure a definite thickness of the respective layers, or bonding layers may be formed between the layers in some cases in order to provide adhesion between them. These also cause the troubles generated by the non-uniformity in thickness of the layers and high cost due to an increase in manufacturing steps.